Adding holes simultaneously within mesh
Select the face you want holes on
Inset all faces by pressing "i" 
Extrude all faces by pressing "e"
Extrude all faces by pressing "e", press "g" and drag them down 
Extrude all faces by pressing "e"


(3. and 5. sharpen the edges, so you don't have to add Loopcuts/Edge Creases)